Hexafluoropropylene has been prepared by the pyrolysis of tetrafluoroethylene. This process has several disadvantages. Tetrafluoroethylene, which is itself difficult to prepare and purify, is an explosive compound, which must be stored and handled with the greatest care. The pyrolysis of tetrafluoroethylene inevitably makes some perfluoroisobutylene as a by-product, and this compound is extremely toxic and is costly to remove and destroy. Another preparative method for hexafluoropropylene is to make it simultaneously with tetrafluoroethylene by pyrolysis of CHClF.sub.2. The product also contains the toxic by-product perfluoroisobutylene, and the process provides a particular mixture of the two products, which may be different from the ratio of products desired by the user. Both of the above synthetic methods are carried out at high temperatures, so it is necessary to make the equipment from rare and expensive metals. Patents describing these processes include U.S. Pat. No. 3,873,630, U.S. Pat. No. 2,970,176, U.S. Pat. No. 3,459,818, U.S. Pat. No. 2,758,138, and U.S. Pat. No. 3,306,940.
U.S. Pat. No. 2,558,703 discloses the Sb-catalyzed fluorination of C.sub.3 Cl.sub.6 to CF.sub.3 CCl.dbd.CCl.sub.2 at 77% yield, but does not mention further fluorination. U.S. Pat. No. 4,680,406 discloses the carbon-catalyzed reaction of C.sub.3 Cl.sub.6 with HF to give, in addition to unreacted starting material and a by-product, C.sub.3 F.sub.5 Cl.sub.3, C.sub.3 F.sub.4 Cl.sub.2, and C.sub.3 F.sub.3 Cl.sub.3. Conditions are limited to a small number of catalysts. U.S. Pat. No. 2,900,423 relates to the synthesis of hexafluoropropylene by hydrogenation of CF.sub.3 --CFCl--CF.sub.3 over a catalyst. The patent gives no information about the washing step or the residual K in the catalyst. No information on catalyst life is presented, the longest run lasting only three hours.
The discovery of an improved process which provides chlorofluorocarbons in high yield in one step from the known C.sub.3 Cl.sub.6 makes possible several reaction sequences for making hexafluoropropylene.